Diesel Motor Oil Guide

Diesel engine oil is subjected to more extreme forms of chemical breakdown, shear, and contamination than certain other engine types. It is these three primary reasons that oil must be changed periodically, as specified by the original equipment manufacturer. Below you will find information regarding oil viscosity in addition to oil capacities, oil spec, and oil change intervals for Ford, General Motors (GMC, Chevrolet), and Dodge/Ram trucks.

Oil Viscosity & Grade

Engine oil viscosity grades identify the lubricants chemical properties. The numbers themselves are not indicative of a single specific property, only that the characteristics fall within the acceptable range of properties for that oil grade (including kinematic and dynamic viscosities) as specified by SAE guidelines. A common misconception is that the "W" found in oil grades is an acronym for "weight", and thus we call a 15W-40 oil "fifteen forty weight". This is incorrect, and the "W" actually stands for "winter" and identifies the oil's cold temperature grade. The second number, following the dash, specifies the oil's hot temperature grade. To maximize engine protection, it is necessary for most engines to require an oil that displays different properties depending on the ambient and/or operating temperature. Manufacturers often specify oil grade requirements based on ambient temperature and these recommendations should be closely followed.

The lower the "weight" or oil grade, the thinner and less viscous the oil is. A low viscosity oil consequently flows easier than a more vicious oil. Multi-grade oils use viscosity modifiers to obtain their specific properties. In addition to an oil's viscosity grade, modern engine oils also include API (American Petroleum Institute) ratings, which requires manufacturers to adhere to specific performance characteristics. The API test requirements include chemical limitations, engine seal compatibility, foaming characteristics, corrosion properties at high temperature, and shear stability. An API certified low ash engine oil, for example, is required in all modern diesel particulate filter equipped trucks.

Oil Change Intervals - Normal vs Severe Duty

OE manufacturers historically provide a "normal" service interval in addition to a "severe" or "heavy duty" interval. In instances that the intervals are not subdivided, the service procedure is unlikely affected by operating conditions or the engine relies on an oil life or oil change monitoring system that alerts the driver when service is needed. Always service an engine as prompted by the oil life/oil life/service procedure monitoring system unless the recommended interval is reached before any indicators are triggered. "Severe" or "heavy duty" maintenance intervals should be followed when operating under certain conditions, which include 1) periods of excessive idling, 2) frequent short trips that do not allow the engine to reach operating temperature, 3) driving on dusty roads, 4) driving off-road or engaging 4 wheel drive, 5) using biodiesel, and most importantly 6) frequent towing/hauling. When in doubt, follow the "severe" duty requirements provided by the manufacturer. Refer to your owners manual for additional driving conditions that may alter your vehicle's maintenance schedule. Never exceed the manufacturer's recommended maintenance intervals for any reason.

Synthetic vs Conventional Oil

Synthetic engine oil is superior every way, shape, and form, although they tend to cost slightly more than a conventional, petroleum based motor oil. Synthetic engine oils are typically produced from mineral oils. The benefits of synthetic motor oils include:

Resilience to chemical breakdown - Motor oil contamination and the corresponding chemical breakdown of engine oil is a major, if not the most profound concern in diesel engines. There are two significant sources of contamination; fuel dilution and soot contamination. Diesel fuel and soot in exhaust gases find there way into the crankcase naturally through blowby. To a degree, fuel dilution is completely normal and unavoidable. However, modern diesel engines tend to suffer more predominantly from fuel dilution, especially those that use a post-injection event as a means of heating the exhaust stream during regeneration cycles. Excessive idling and cold weather operation also contribute to greater degrees of fuel dilution as a result of cylinder washing do to relatively cool combustion temperatures. Fuel dilution lowers the viscosity of engine oil, making it thinner. Soot contamination increases viscosity, making the oil thicker. Both have unfavorable effects on the properties of engine oil, thus a synthetic oil with a high resilience to these contaminants is desirable.

Resilience to thermal breakdown - Most diesel engines have a normal operating temperature of 185 - 205 ° F. This means that, at best, your engine oil is experiencing temperatures in this range. But it's not normal engine temperature that is a huge concern with regard to thermal breakdown, especially considering that most diesels utilize an oil cooler. The turbocharger, however, produces a tremendous amount of heat that is removed by pumping engine oil through it. A synthetic motor oil is far more resilient to high temperatures and breaks down at a significantly slower rate, thus maintaining its fluid properties between oil change intervals.

Resilience to mechanical breakdown (shear) - Engine oil experiences intense shear forces in any engine, regardless of fuel type. Engine oil must create a barrier between two or more metal surfaces in motion, absorbing the forces that are transmitted between gears, bearings, rods, and pins. The extreme force and pressure that is created between these surfaces contributes to oil breakdown. This form of breakdown is typically referred to as shear, and once again synthetic oils are much more resilient than conventional petroleum based oils.

It's never too late to switch to synthetic motor oil and it is completely compatible with conventional petroleum oils. There is therefore no special process required when converting to synthetic oils; simply drain and refill. Synthetic engine oils, despite a marginally higher price point, are becoming an industry standard as a result of their superior characteristics.

Oil Recommendations

Amsoil Inc. is arguable the most renowned name in the synthetic motor and gear oil marketplace. Founded in 1972, the company has been at the forefront of technology in the synthetic oil sector. Their line of diesel engine oils are specifically formulated to endure the severe conditions your engine oil encounters. Their most recent line of diesel engine oils is organized into two tiers - Signature Series and Heavy Duty. The Signature Series product line promises 6x more wear protection than required by Detroit Diesel's DD13 Scuffing Test, while the Heavy Duty series engine oil advertises 4x more wear protection. All the project vehicles that enter our shop leave with Amsoil, bumper-to-bumper. For pricing and additional information, see the product links below.

Disclaimer: Diesel Hub provides no warranty as to the accuracy, in part or in full, of the content published herein. Diesel Hub expressly disclaims all liability for damages of any kind arising from use, reference to, or reliance of any information published herein. This includes, but is not limited to vehicle damage and personal injury. Personal protective equipment (PPE) is advised when performing any automotive service/repair procedure(s) to avoid contact with hazardous materials and/or minimize the risk of injury.